Supporting Information Epitaxial Growth of Twinned Au-Pt Core-Shell Star-Shaped Decahedra as Highly Durable Electrocatalysts Ting Bian,, Hui Zhang,,* Yingying Jiang, Chuanhong Jin, Jianbo Wu, &,* Hong Yang,,* and Deren Yang State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, and Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou, Zhejiang 310027, People s Republic of China & State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd, Shanghai, 200240, People s Republic of China Department of Chemical & Biomolecular Engineering, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, 114 Roger Adams Laboratory, Urbana, Illinois 61801, USA *Correspondence to: msezhanghui@zju.edu.cn, jianbowu@sjtu.edu.cn, hy66@illinois.edu.
Experimental Section Chemicals and Materials. Chloroauric acid tetrahydrate (HAuCl4 4H2O, Sinopharm, 99.9%), hexachloroplatinic acid (H2PtCl6, Sigma-Aldrich, 99.9%), oleylamine (OAm, Aladdin), hexadecyltrimethyl ammonium bromide (CTAB, Sinopharm, 99%), trioctylphosphine oxide (TOPO, Sigma-Aldrich, 99%), hexadecyltrimethyl ammonium chloride (CTAC, Sinopharm, 98%), didodecyldimethylammonium (DDAB, Aladdin, 98%), octadecylamine (Sigma-Aldrich, 99%), and tetrahydronaphthalene (Sigma-Aldrich, 99%) were all used as received. All syntheses were carried out in one-neck flask (25 ml). One-Pot Synthesis of Au-Pt Core-Shell Star-Shaped Decahedra. In a standard procedure, 300 mg of CTAB, 100 mg of TOPO and 6 ml of OAm were mixed in a flask and the mixture was heated to 180 C in air under magnetic stirring. Meanwhile, 12.4 mg of HAuCl4 and 15.5 mg of H2PtCl6 were dissolved into 3 ml of OAm, resulting in a yellow precursor solution. After that, the precursor solution was rapidly injected into the hot solution containing CTAB and TOPO with a pipette. The reaction was allowed to continue at 180 ºC for 3 h in air. Finally, the solution was centrifuged and washed three times with ethanol and hexane to remove CTAB, TOPO and OAm before characterization. We systematically investigated the effects of various parameters, including the duration of reaction, the amount of CTAB and TOPO, the type of solvent, as well as the temperature, on the morphology of resultant Au-Pt nanocrystals. Two-step Synthesis of Au-Pt Core-Shell Star-Shaped Decahedra. Au-Pt core-shell star-shaped decahedra were synthesized by seeded growth with Au decahedra as seeds. Firstly, 3 ml of OAm containing 12.4 mg of HAuCl4 was rapidly injected into a mixture of OAm (6 ml), CTAB (300 mg), and TOPO (100 mg) at 180 C under magnetic stirring in an effort to prepare Au decahedral seeds. After the solution changed to a deep purple color within 5 min, the products were collected by centrifugation and then washed one time with ethanol and hexane. Secondly, the as-prepared Au seeds were dispersed in 6 ml of OAm containing 300 mg of CTAB and 100 mg of TOPO. The solution was pre-heated to 180 C under magnetic stirring for 15 min. Subsequently, 3 ml of OAm solution containing 15.5 mg of H2PtCl6 was rapidly injected into the pre-heated solution with a pipette. After that, the reaction was allowed to continue at 180 C for 3 h. Finally, the solution was centrifuged and washed three times with ethanol and hexane to remove excess CTAB, TOPO and OAm for further applications. S2
Morphological, Structural and Elemental Characterizations. Transmission electron microscopy (TEM) images of the obtained samples were taken using a Philips CM 200 microscope operated at 200 kv. High-resolution transmission electron microscopy (HRTEM) was performed using a FEI Tecnai F30 G2 microscope operated at 300 kv. High-angle annular dark-field scanning TEM (HAADF-STEM) and Energy dispersive X-ray (EDX) mapping analyses were taken on a FEI Titan ChemiSTEM equipped with a probe-corrector and a Super-X EDX detector system. This microscope was operated at 200 kv with a probe current of 50 pa and a convergent angle of 21.4 mrads for illumination. The percentages of Au and Pt in the samples were determined using inductively coupled plasma mass spectrometry (ICP-MS, Perkin-Elmer Elan DRC IIICP-MS). Extinction spectra of the samples were recorded using a UV-vis spectrometer (Hitachi UV-4100). Preparation of Carbon-Supported Catalysts. Carbon black (Vulcan XC-72) was used as support for making Au-Pt catalysts (Au-Pt/C). [1] In a standard preparation, carbon black particles were dispersed in chloroform and sonicated for 30 min. The as-prepared Au-Pt nanocrystals (20 wt%) were added to this dispersion. This mixture was further sonicated for 10 min and stirred for 12 h. After that, the solids were precipitated out by centrifugation and re-dispersed in n-butylamine at a concentration of 0.5 mg-catalyst/ml. The mixture was kept under magnetic stirring for 3 days, and then centrifuged and washed three times with methanol. Electrochemical Measurement. A three-electrode cell was used to measure the electrochemical performances of Au-Pt/C catalysts including the commercial Pt/C (ETEK). A glassy-carbon rotating disk electrode (RDE) was used as the working electrode (area: ~0.196 cm 2 ). A 1 cm 2 platinum foil and a HydroFlex hydrogen electrode were used as the counter electrode and the reference. The reference electrode was placed in a separate compartment connected with main cell via a salt bridge. The hydrogen reference electrode was calibrated before all the tests via Hydrogen evolution reaction (HER). All potentials were referenced to the reversible hydrogen electrode (RHE). The electrolyte for cyclic voltammetry (CV) measurement and linear scan voltammetry (LSV) test for oxygen reduction reaction (ORR) was 0.1-M HClO4 solution, diluted from 70% double-distilled perchloric acid (GFS Chemicals, USA) with Millipore ultrapure water (18.2 MΩ). To make catalyst ink, 5 mg of Au-Pt/C catalysts was dispersed in 10 ml of a mixed solvent and sonicated for 10 min. The solvent contained a mixture of de-ionized water, isopropanol, and 5% Nafion 117 solution at the volumetric ratio of 8:2:0.05. 30 μl of the catalyst S3
ink was added onto the RDE and dried under the air flow for 30 min to make the working electrode. The loading amount of Au-Pt core-shell catalysts on the RDE was determined to be ~15 μgmetal/cm 2. The electrochemical active surface area (ECSA) was determined from the CV curves, calculating the amount of charges by integrating hydrogen desorption region after double layer correction. The CV measurement was carried in argon-saturated 0.1 M HClO4 solution at room temperature with a scan rate of 50 mv/s. ORR LSV cures were measured at the rotating rate of 1600 rpm in a 0.1-M HClO4 solution, which was purged with oxygen for 30 min prior to, and during testing. The scan rate for ORR measurement was set at 10 mv/s. Data were used after ir-drop correction. The accelerated stability test (ADT) was carried out between 0.6 V and 1.0 V at a scan rate of 100 mv/s 1 for 30,000 cycles in oxygen saturated 0.1-M HClO4 solution. (1) Wu, J.; Zhang, J.; Peng, Z.; Yang, S.; Wagner, F.; Yang, H. J. Am. Chem. Soc. 2010, 132, 4984. S4
Table S1. ICP-MS Data of the Au-Pt Star-Shaped Decahedra and Spherical Nanoparticles with the Core-Shell Structures. sample nanoparticle catalyst shape nau:npt* Au/Pt atomic ratio composition 1:1 1:1.03 AuPt1.03 1:0.9 1:0.79 AuPt0.79 star-shaped decahedral 1:0.8 1:0.68 AuPt0.68 1:0.7 1:0.66 AuPt0.66 1:0.6 1:0.62 AuPt0.62 1:0.5 1:0.4 AuPt0.4 spherical 1:1 1:1.08 AuPt1.08 *: Molar ratio between the Au and Pt salt precursors S5
Table S2. Specific ECSA, Area and Mass Activity of Au-Pt Star-Shaped and Pt/C Catalysts. sample specific ECSA (m 2 /gmetal) specific area (ma/cm 2 ) activity** total metal mass (ma/μgmetal) Pt mass (ma/μgpt) AuPt0.4 46.2 0.20 0.09 0.33 AuPt0.62 48.8 0.39 0.19 0.50 AuPt0.66 48.1 0.55 0.26 0.65 AuPt0.68 46.6 0.78 0.36 0.89 AuPt0.79 48.4 0.88 0.42 0.97 AuPt1.03 43.5 1.09 0.47 0.94 AuPt NC 52.3 0.18 0.10 0.19 Pt/C 71 * 0.2 0.14 0.14 *: Specific ECSA value of Pt/C was obtained from Figure S15. **: All activity values were determined at 0.9 V vs. RHE. S6
Table S3. Durability Study of AuPt0.4 and AuPt1.03 Star-Shaped, AuPt1.08 NP, and Pt/C Catalysts. sample specific ECSA (m 2 /gmetal) specific area (ma/cm 2 ) activity* total metal mass (ma/μgmetal) Pt mass (ma/μgpt) AuPt0.4 star 46.2 0.20 0.09 0.33 After 30k 28.1 0.18 0.050 0.18 AuPt1.03 star 43.5 1.09 0.48 0.94 After 30k 38.1 0.97 0.37 0.72 AuPt1.08 NC 51.4 0.18 0.0925 0.185 After 30k 33.2 0.15 0.0483 0.093 Pt/C 71 0.2 0.14 0.14 After 30k 38.1 0.15 0.057 0.057 *: All activity values were determined at 0.9 V vs. RHE. S7
Figure S1. Schematic illustration the definition of size for a star-shaped decahedron. S8
Figure S2. TEM image of the Au-Pt nanocrystals prepared using the standard procedure. The inset shows a magnified micrograph of an individual star-shaped decahedron. S9
Figure S3. TEM images of the Au-Pt star-shaped decahedral nanocrystals recorded at different tilting angles: (a) 0 o, (b) -30 o along the X-axis direction, (c) 30 o along the Y-axis direction, (d) -30 o along the Y-axis direction. S10
Figure S4. UV-vis spectra of the products collected at different time in a standard procedure. The inset shows a photograph of the corresponding samples. S11
Figure S5. TEM images of (a) the Au seeds and (b) the Au-Pt star-shaped decahedra in the two-step synthesis. S12
Figure S6. The variation of the thickness of Pt shells with different molar ratios of Pt to Au precursors. The inset illustrates schematically the definition of size for Pt shell thickness (Δd) in a star-shaped decahedron. S13
Figure S7. (a) High and (b) low magnification TEM images of Au-Pt star-shaped nanoparticles prepared using the standard procedure except for the reaction time of 8 h. S14
Figure S8. TEM images of Au-Pt nanocrystals prepared using the standard procedure, except for the difference in the type and amount of the solvents: (a) 8.5 ml of tetrahydronaphthalene and 0.5 ml of OAm, (b) 6.75 ml of tetrahydronaphthalene and 2.25 ml of OAm, and (c) 9 ml of octadecylamine. S15
Figure S9. TEM images of the Au-Pt nanocrystals prepared using the standard procedure, except for different temperatures: (A) 140, (B) 160, (C) 190, and (D) 200 ºC. S16
Figure S10. TEM images of Au-Pt nanocrystals that were prepared using the standard procedure, except for the difference in the amounts of CTAB and TOPO added in the reaction mixtures, or replacing CTAB with CTAC and DDAB: (a) in the absence of TOPO, (b) in the absence of CTAB, (c) 30 mg of CTAB and 100 mg of TOPO, (d) 100 mg of CTAB and 100 mg of TOPO, (e) 300 mg of CTAC and 100 mg of TOPO, and (f) 300 mg of DDAB and 100 mg of TOPO. S17
Figure S11. (a) TEM image, (b) HRTEM image, (c) HAADF-STEM-EDX line-scan, and (d) HAADF-STEM-EDX mapping analysis of the Au-Pt nanocrystals prepared using the standard procedure except for the absence of CTAB. S18
Figure S12. CV curves of AuPt star-shaped catalysts with various compositions and AuPt NC. S19
1st cycle 30,000th cycle Figure S13. Polarization curves for ORR durability test of Pt icosahedral catalyst recorded after 1st (black color) and 30,000th (red color) cycle, respectively. S20
Figure S14. TEM and HAADF-STEM-EDX mapping images of the carbon-supported (a, b) AuPt1.03/C and (c, d) AuPt0.4/C catalysts after ADT for 30,000 cycles. S21
Figure S15. CV curve of reference Pt/C catalyst. S22